2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Fluid enrichment processes in origin of ore mineralization for felsitic veins of the Platreef,

Bushveld Complex, South Africa

Zhitova L.M.***, Borovikov A.A.**, Gora M.P.**, and Lapkovsky A.A.**

*Novosibirsk State University, Novosibirsk, Russia;

**V.S. Sobolev Institution of Geology and Mineralogy SB RAS, Novosibirsk, Russia

zhitova@igm.nsc.ru

Felsitic veins composed of quartz-feldspar symplectites cross-cut ultramafic rocks of the Platreef (Armitage et al., 2002). These veins contain a number of accessory minerals included in quartz: sulphides, REE-bearing Ir-Os-alloys, cooperite (PtS), native silver, zircon, monazite, U-Th- and Nb-Ti-bearing phases (Hutchinson, Kinnaird, 2005). According to ICP-MS and XRF analyses the veins are enriched in metals: 230 ppb Au, 1440 ppb Pt, 1665 ppb Pd, 2826 ppm Cu and 2063 ppm Ni. It has been considered that felsitic liquids originated from prolonged crystallization of magmas and/or contamination of a mafic melt by the sedimentary country rocks or the basement granitic gneiss (Kinnaird et al., 2005). The formation of the felsitic veins, defined by a zircon age of 2053.7 ± 3.2 Ma, corresponded to the age of main magmatic activity at the Bushveld Complex (Kinnaird et al., 2005). The felsitic liquids and related fluids, while percolating through the Platreef rocks, are thought to be responsible for redistribution of initial magmatic sulphides and PGM (Armitage et al., 2002; Kinnaird et al., 2005). This idea was tested in our study of fluid and sulphide inclusions hosted in quartz and calcite from the Platreef felsitic veins. We used the methods of thermometric studies, Raman-spectroscopy, scanning electron microscopy, EMPA and laser ablation ICPMS.

The samples of the felsitic vein with quartz-feldspar symplectites were collected from the Sandsloot open-pit mine and Malatzane Stream, as well as the borehole cores of the Tweefontain Section of the Platreef. Most quartz grains are characterized by disseminated sulphides, REE-phosphates and numerous small fluid inclusions (< 5 µm), unsuitable for detailed studies. Samples of quartz and calcite from a large miarolitic cavity in the pegmatitic quartz-feldspar vein (SS2) in the Sandsloot open-pit mine were found most informative to our study. Quartz SS2 is formed by large (10 x 8 cm) prismatic, semi-transparent and zoned Brazilian twin crystals of the first generation, overgrown by clear small crystals (up to 1-3 cm). Calcite SS2 forms blocky aggregates intergrown with quartz. Calcite shows zones enriched in disseminated sulphide inclusions.

The results of our fluid inclusion study in the SS2 quartz are presented in Table 1. Primary multiphase inclusions (L+V+H+1-4S) and coeval vapour-rich inclusions with liquid CO₂ are present in the inner zones of the first generation quartz and form clusters that sub parallel twinning. Primary two or three-phase inclusions and syngenetic CO₂-CH₄ gaseous inclusions decorate outer growth zones of this quartz. The second generation quartz hosts aqueous saline inclusions and CH₄-bearing vapour inclusions at the base of the crystals only, so other parts of grains are very clear in appearance. Most fluid inclusions are 10-20 µm, and some are up to 30-50 µm. The multiphase inclusions usually decrepitated at 250-300°C, before complete homogenization. Similar inclusions have been described in miarolitic and symplectitic quartz of the Merensky Reef (Borisenko et al., 2006). We succeeded in dissolving vapour bubble at 220-200°C in the smaller size multiphase inclusions, however, melting of solid phases were not observed before decrepitating, except partial dissolution of halite at 550°C. If all solid minerals in these inclusions belong to daughter phases, then the total salinity of their parental fluid would be extremely high (~80 wt% NaCl eq.), which seems unlikely. Scanning electron microscopy of opened multiphase inclusions recorded a number of minerals, such as NaCl, KCl, CaCl₂, MnCl₂, CaCO₃, BaSO₄, REE-aluminosilicate, and REE- phosphate.

Syngenetic fluid inclusions with liquid CO₂ are represented by a type L+LCO₂+V. The temperatures of CO₂ melting and homogenisation are -57.8/-59.6 and +29.6/+31.0°С, respectively. Homogenization occurred in the vapour phase in most cases, and sometimes a critical behaviour was observed. The pressure of 0.74 kbar was estimated using the average CO₂ density of 0.34 g/cm³ and inferred trapping temperature of 650°С. Complete homogenization of these inclusions occurred at 240-235°С. The results of the Raman-spectroscopic studies of fluid inclusions suggest the evolution of gaseous phase from essentially CO₂ (first generation quartz) to essentially CH₄ (second generation quartz). The bulk fluid composition evolved from high-temperature oxidized brines with CH₄-CO₂ vapour towards cooler and more reduced saline aqueous solutions with CH₄-rich vapour.

The calcite SS2 contains primary two-phase aqueous fluid inclusions (L>>V), belonging to certain growth zones, where they coexist with inclusions of sulphide minerals. Sulphide inclusions in calcite are represented by platelet-like pyrrhotite and dendritic crystals of chalcopyrite. The morphology of chalcopyrite grains often reflects cleavage planes of host calcite. Same fluid is found around sulphide inclusions. These aqueous inclusions homogenize into liquid at 75-120°С. When deep freezing (up to -190°C) the liquid phase of inclusions becomes metastable (glassy), and its crystallization to skeletal ice crystals occur at -98°C. The temperature of ice melting is -54°C for all the inclusions. The beginning of ice melting occurs at -74°C that is the eutectic temperature.

The composition of fluid inclusions in the SS2 quartz (first generation) was studied in the University of Tasmania using LA-ICPMS (New Wave UP-183 laser and AGILENT 7500 mass-spectrometer). Among different types of analysed inclusions (multiphase L+V+H+1-4S with 29-36% salinity, four-phase L+V+H+S with 30-32% salinity and three-phase L+V+H with 21-23% salinity) the most saline multiphase inclusions are characteristically more metal-enriched. They contain elevated Mn, Fe, Pb, and also Co, Ni, Cu, As, Mo, Sn, Sb, Bi The PGE (Pt and Pd) are not present above the detection limit. All compositions have a deficit of Cl relative to major cations (Na, K and Ca), which suggests presence of other anions (e.g. carbonate). These data confirm the possibility of metal re-distribution by the late magmatic fluids. High Pb abundances (1000’s ppm) in the fluid inclusions permitted quantification of Pb isotope composition, including minor isotope ²⁰⁴Pb. Comparison of Pb isotope ratios in the fluid inclusions with the crustal Pb growth trend produced an age estimate of 2000±70 Ma (c.f. 2054±22 Ma age of the Bushveld complex (Kinnaird et al., 2005). Application of the same method to sulphide inclusions in the SS2 calcite showed presence of significant concentrations of Ag, Hg, Sn, Se, As, Ni, Co, and Zn, but the PGE were not confidently detected. The concentration of Ag was higher in chalcopyrite than in pyrrhotite, which is probably related to presence of microcrystals of native Ag in chalcopyrite. We consider formation of sulphides from a suspension of sulphide particles in aqueous liquid, and trapping along the growth planes of simultaneously crystallizing calcite crystals. Composition of fluid inclusions in calcite by ICPMS data showed the presence of Na, K, Ba, Rb, Sr, Ca, Cl as well as of Cu, Pb, Zn, Cr and traces of Ag, Au, Sb, Hg. The presence of Zn in solution probably as ZnCl₂ phase is responsible for low (-74°C) eutectic temperature, although this is also typical for LiCl-bearing water-salt systems.

The study of fluid and crystalline inclusions in quartz and calcite from the miarolitic cavity of the pegmatite symplectitic vein resulted in new estimates of physical and chemical characteristics of magma-related fluids at a postcumulus stage of the Platreef formation. The most primitive fluids were trapped by quartz at 600-650°С and >2 kbar. They were represented by heterogeneous oxidised carbonate-chloride solutions (20-80 % salinity) with the methane-carbon dioxide gaseous phase. These fluids evolved into lower temperature, more reduced aqueous solutions with less salinity (< 25%) and the methane gaseous phase. All the above fluid components were immiscible with each other. High-temperature brines carried metals, including high concentrations of Pb, which isotope composition corresponded to crustal Pb at the time of formation of the Platreef symplectitic veins and the Bushveld complex. Later fluids had lower temperature (75-120°С), as recorded by fluid inclusions in calcite coexisting with the miarolitic quartz, and were also metal-bearing. Possibly, metals were present in the sulphide suspension in the aqueous fluid and released into sulphides during calcite crystallisation, whereas residual fluid was co-trapped with sulphide minerals in the form of fluid inclusions. High concentrations of metals in quarts-hosted fluid inclusions indicate significant role of carbonate-chloride aqueous fluids in the re-distribution of metallic elements, including the PGE, primarily deposited in the magmatic process.

This study was financially supported by RFBR grant 11-05-00681-a.

References:

Armitage P.E.B., McDonald I., Edwards S.J., Manby G.M. Platinum-group element mineralization in the Platreef and calc-silicate footwall at Sandsloot, Potgietersrus, District, South Africa //Trans. Inst. Min. Metall., Appl. Earth Sc. 2002. V. 11. B36-B45.

Kinnaird J.A., Hutchinson D., Schurmann L., Nex P.A.M., R. de Lange Petrology and mineralization of the southern Platreef: northern limb of the Bushveld Complex, South Africa // Miner. Dep. 2005. V.40. P. 576-597.

Hutchinson D., Kinnaird J.A. Complex multistage genesis for the Ni-Cu-PGE mineralization in the southern region of the Platreef, Bushveld Complex, South Africa // Trans. Inst.Min.Metall., Appl. Earth Sc. 2005. V. 114. B208-B224.

Borisenko A.S., Borovikov A.A., Zhitova L.M., Pavlova G.G. Composition of Magmatogene fluids and factors determining their geochemistry and metal contents // Rus. Geol. and Geophys. 2006. V.47 (12). P. 1308-1325.